Field of the Invention
Embodiments of the present invention generally relate to a method to lessen the impact of a switch in a micro electromechanical system (MEMS) device.
Description of the Related Art
A digital variable capacitor (DVC) operates with electrostatic forces. In this mechanism, a force is acting on the moveable MEMS device when a voltage V is applied between the MEMS device and a control electrode. This electrostatic force scales with (V/gap)2. The mechanical counter-balance force comes from a spring suspension system and typically scales linearly with the displacement. The result is that with an increasing voltage V the MEMS device moves a certain distance δ toward the control-electrode. This movement reduces the gap which in turn increases the electrostatic force further. For small voltages, an equilibrium position between the initial position and the electrode is found. However, when the voltage exceeds a certain threshold level (i.e., the pull-in voltage), the device displacement is such that the electrostatic force rises faster than the mechanical counterbalance force and the device rapidly snaps-in (i.e., moves) towards the control-electrode until it comes in contact with a contact surface.
Some DVC devices have a control-electrode above (i.e., a pull-up or pull-off or PU-electrode) and below (i.e., a pull-down or pull-in or PD-electrode) the moveable MEMS device (i.e., the plate in FIG. 1), as shown schematically in FIG. 1. In addition there is an RF-electrode below the moveable MEMS device. During operation the MEMS device is either pulled-up or pulled-down in contact to provide a stable minimum or maximum capacitance to the RF-electrode. In this way the capacitance from the moveable device to the RF-electrode (which resides below the moveable device) can be varied from a high capacitance Cmax when pulled to the bottom, as shown in FIG. 2, to a low capacitance Cmin when pulled to the top, as shown in FIG. 3.
In production, the MEMS devices will exhibit a variation in pull-in voltages due to manufacturing tolerances, such as layer thicknesses and stress levels. In addition, some MEMS devices require a certain overvoltage beyond the pull-in voltage to be applied to the pull-in electrode to provide for a stable capacitance. Additionally, the CMOS controller will exhibit some variation in the available voltage levels due to manufacturing tolerances as well. As a result, the voltage levels applied to the pull-electrodes are typically designed such to provide for enough margin against manufacturing tolerances.
If this high voltage level is applied to the pull-in electrode very quickly, the MEMS device will rush towards the pull-in electrode very quickly because the MEMS device immediately sees an electrostatic force much larger than required for pull-in. This will lead to an acceleration of the MEMS device towards the pull-electrode from the start and result in a high impact velocity causing damage to the contact surfaces.
Therefore, there is a need in the art for reducing the impact of the MEMS devices on the contact surfaces.